One type of display used in digital image display systems, including digital television systems, is a spatial light modulator (SLM). A spatial light modulator consists of an array of electronically addressable pixel elements and related control circuits. A frequently used type of SLM is the digital micro-mirror device in which each pixel element is a tiny micro-mechanical mirror, capable of separate movement in response to an electrical input. Incident light may be modulated in direction, phase, or amplitude for reflection from each pixel.
In a typical application, the source of incident light is a light source filtered by a transparent color wheel composed of the three primary colors of light, red, green, and blue. This motor driven wheel rotates such that the light source is projected through each color segment during one rotation. Because pixel elements in an SLM are "binary" in the sense that they are either on or off, various pulse width modulation techniques may be used with the color wheel to display a color image. These techniques are described in pending U.S. Pat. No. 5,278,652 entitled, "DMD Architecture and Timing for Use in a Pulse Width Modulated Display System" assigned to Texas Instruments Incorporated. This application is incorporated by reference as if set forth fully herein. Because a color wheel contains the primary colors of light, a variety of colors may be displayed by displaying the appropriate primary colors for a required period of time.
A continuous video image, such as a television video signal, consists of a series of video frames. To properly display one frame of an image, the color wheel must make at least one rotation during each frame in a video image. To display the proper colors for a video image, a the rotation of the color wheel and the beginning of a video frame should be synchronized.